They vary A LOT. Depends on what you're buying really. If you're Australian. try this site www.fitsmart.com.au for its food database. It has most of the frozen meals in it. From the brief time i looked at them the range anywhere from 250 cal to 650 cal per serve.

If you're referring to Healthy Choice, Lean Cuisine, etc., I'd have to say no. Those meals tend to be full of sodium, preservatives, etc. While I'd say they're okay to have in a pinch, I wouldn't recommend eating them too frequently.

They are surely very convenient and are indeed healthy, unless you have hypertension. Portion control and controlling the macro-nutritional composition of food is extremely difficult, especially for busy people, so convenient meals like Lean Cuisine, Smart Ones, and Healthy Choice are excellent alternatives.

I eat two of 'em a day and have lost almost 40 pounds, so it can be done! I don't eat high sodium foods the rest of the day and I still fall around 2 gm NA (which is under the recommended max so I'm still OK there). I exercise a bit of that salt out as well.

Cooking your own meals is, of course, optimal, and folks who have the time and energy to do so should do so as much as possible. Typically, folks choose meal replacement entrees because they don't have the time or energy, and need the convenience. Choosing based on protein content, as Susan alludes to, is a very good idea. I try to evaluate all food choices based on protein grams per calorie. Some Lean Cuisines and such are good with respect to protein -- others aren't. Always read labels! However, for folks trying to lose weight, or keep it off, meal replacements are almost always healthier choices than high-fat or high-calorie alternatives.